Microfilm strips for use in a microfilm reader/printer apparatus or the image frames on such a microfilm strip include those given in the form of negatives or those given in the form of positives. In whichever form the microfilm strips or the image frames may be given, it is preferable that the images thereon be reproduced or printed in positive. It is for this reason necessitated in a microfilm reader/printer apparatus to discriminate whether the microfilm strip or the image frame of the microfilm strip currently in use is a negative or a positive and to establish operational conditions adapted for the printing of the image in positive.
Such discrimination of a microfilm strip or an image frame between negative and positive may be effected by visual inspection of the microfilm strip or the image frame. Discrimination by human intervention is reliable in the result but is not acceptable for high-speed operations. Mechanized and otherwise automated negative/positive microfilm discriminating systems have therefore been proposed and put to use. Known examples of such systems are disclosed in Japanese Patent Specification No. 49-16647 and U.S. Pat. No. 4,341,463.
The negative/positive microfilm discriminating system taught in Japanese Patent Specification No. 49-16647 utilizes the fact that the density in a non-image area between image areas of an image frame differs between negative and positive film strips. An image frame is thus discriminated to be a negative or a positive through detection of the density of such a non-image area of the frame.
On the other hand, the microfilm discriminating system disclosed in U.S. Pat. No. 4,341,463 illuminates a particular area of an image frame of a microfilm strip to detect the density of the background area (viz., the base density of the microfilm strip) of the image frame from the quantity of the light transmitted through the particular area of the frame. The density of the background area of the image frame thus detected is compared with a prescribed reference value for determining whether the image frame is a negative or a positive.
A known negative/positive microfilm discriminating system of the former type has a problem in that the result of the discrimination may be erred since the discrimination is made on the basis of the density in a non-image area of an image frame, not directly from the image on the frame. In a prior-art system of the latter type, discrimination of an image frame between negative and positive is made directly from the image on the frame and the likelihood of the system making an erred decision will be less than that in a system of the former type. The system of the latter type however has a drawback in that, when there is a stain or dust on a microfilm strip or in the optical arrangement of the system, the data representative of the density distribution of the image will contain information relating to the density of such a stain or dust. The use of such data for the discrimination of the image frame between negative and positive would result in an erred decision. In a known negative/positive microfilm discriminating system of either type, no consideration is given to the presence of a stain or dust on a microfilm strip or in the optical arrangement of the system and, accordingly, the effect resulting from the presence of a stain or dust is unavoidable in the system.